After a peak in 2005, infant death rates have recently been on the decline. In 2013, they were down 13 percent. And now, this CDC report shows a decrease of another 2.3 percent in 2014, to a new low of 582.1 infant deaths per 100,000 live births, from 596.1 the year before.

"This is potentially the best news we've had yet," CDC's T. J. Mathews, who has written extensively about infant mortality, told The New York Times.

While researchers have not yet pinpointed a specific reason for the decline, a decrease in respiratory distress syndrome—which dropped 14 percent last year—may play a role.

According to Eugene R. Declercq, a health researcher at Boston University, the practice of scheduling C-sections early and inducing labor early was believed to have been a potential contributor to infant mortality, and there had been a push among doctors to stop inducing births or scheduling C-sections before 39 weeks of gestation unless there was a strong medical reason to do so. "There's been a conscious effort to change practices," he said. "It has been one of the success stories."

Despite the decline, however, experts said the numbers are still relatively high when compared to the rates calculated in other countries. "Its good news that the infant mortality rate dropped last year," Dr. Steven Woolf, director of the Center on Society and Health at Virginia Commonwealth University told The New York Times, "but it's still much higher than the average of all the countries."